Seismogenic structure beneath the northern Longitudinal Valley revealed by the 2018–2021 Hualien earthquake sequences and 3-D velocity model

Abstract

This study uses the 3D crustal velocity model and the relocated earthquake hypocenters, including the 2018–2021 earthquake sequences, to re-assess the seismogenic structures at the northern Longitudinal Valley. Earthquake focal mechanisms and relocated hypocenters from earthquake clusters suggest a gentle west-dipping fault existing under the Longitudinal Valley and the Coastal Range. Earthquake clusters associated with this west-dipping fault indicate it develops along the base of high-velocity Central Range metamorphic rocks and is likely branched out from the previously recognized Central Range Fault (CRF). Both the 3D velocity model and the geometry of earthquake clusters suggest this structure truncates the Longitudinal Valley Fault north of 23.7° N, separating the northernmost LVF into the shallow and the deep segments. The shallow segment then plausibly evolves to be a transpressional fault system that mainly accommodates the left-lateral motions. This interpretation coincides with the geomorphological and geodetic observations showing that the northern LVF is dominated by the left-lateral faulting, instead of showing a significant reverse component as in the southern Longitudinal Valley. The limited fault width and geometry of the shallow LVF segment also imply its seismic potential is relatively limited, while the underling west-dipping fault and the deeper segment of the LVF are the major seismogenic structure. Such development of the major CRF-related west-dipping structure could accommodate the northwestward subduction of the Philippine Sea Plate and also likely reactivate part of the Offshore Eastern Taiwan Thrust Belt.